Linear low density polyethylene (“LLDPE”) copolymers that are prepared by the copolymerization of ethylene with a higher alpha olefin using a conventional Ziegler-Natta type catalyst system are known to contain three different polymer fractions (or “modes”), namely 1) a low molecular weight fraction which contains a high comonomer content; 2) a copolymer fraction of intermediate molecular weight and intermediate comonomer content; and 3) a high molecular weight fraction which contains little or no comonomer. The low molecular weight fraction is sometimes also described as being “highly branched” (due to the high comonomer content) and/or “grease” (due to the low molecular weight). The high molecular weight content is also sometimes described as “homopolymer”. The “grease” fraction often causes organoleptic problems and may even limit or restrict uses in which the polymer comes in contact with food. The “homopolymer” fraction generally imparts a stiffness to the resin and melts at a higher temperature than the other fractions. In general, the non-uniformity of the molecular weight and the comonomer content is a distinguishing characteristic of conventional Ziegler resins. It is now common to refer to these resins as “heterogeneous”.
In contrast, “homogeneous” polyethylene copolymers are generally characterized by having a narrow molecular weight distribution and a narrow composition distribution. The term “homogeneous” was proposed by one of us to describe such polymers in U.S. Pat. No. 3,645,992 (Elston), the disclosure of which is incorporated herein by reference.
As noted in Elston '992, homogeneous polymers have a distinct melting point due to the uniform polymer architecture. The homogeneous polymers disclosed in the Elston '992 patent were prepared with a vanadium catalyst system which is insufficiently active to permit widespread commercial use.
Advances in catalyst technology now permit the production of homogeneous ethylene copolymers at commercially viable rates. For example, the metallocene/aluminoxane catalysts disclosed by Kaminsky (U.S. Pat. No. 4,542,199) and improved by Welborn (U.S. Pat. No. 5,324,800); the monocyclopentadienyl catalysts disclosed by Stevens et al. (U.S. Pat. No. 5,064,802) and Canich (U.S. Pat. No. 5,055,438); the ketimine catalysts disclosed by McMeeking et al. (U.S. Pat. No. 6,114,481); and the phosphinimine catalysts disclosed by Stephan et al (U.S. Pat. No. 6,063,879) are all highly active for the preparation of homogeneous copolymers.
A particularly important end use of LLDPE is the manufacture of films. Films prepared from homogeneous LLDPE generally have good optical properties, good organoleptic properties and excellent impact strength.
However, films prepared from homogeneous LLDPE generally have poor tear strength, particularly in the so-called “machine direction”. In addition, homogeneous resins are difficult to “process” (i.e. to convert to films). This poor processability is manifested by high energy demands required to extrude the resin (e.g. large current draws on the electric motors used to drive the extrudes) and/or poor melt strength.
Attempts have been made to blend heterogeneous (Ziegler-Natta, or “Z/N”) resins with homogeneous resins in order to produce a resin blend which is easier to convert to film and/or to produce film having higher impact strength and good tear properties. U.S. Pat. No. 5,530,065 (Farley, to Exxon) teaches that a trivial blend of a conventional heterogeneous Z/N resin and a metallocene resin has a balance of properties which are suitable for some film properties. Similarly, U.S. Pat. Nos. 5,844,045 and 5,869,575 (Kolthamer, to Dow) also disclose that simple blends of a conventional heterogeneous Z/N resin and homogeneous resin prepared with a monocyclopentadienyl catalyst are also suitable for preparing films.
However, it will also be appreciated that the simple blends of the above '065, '045 and '575 patents all contain the low molecular weight “grease” due to the use of the Z/N catalyst to prepare some of the blend composition. Moreover, the disclosures of the '065, '045 and '575 patents are silent with respect to the hexane extractables contents of the blends.
Thus, films prepared from conventional heterogeneous resins have comparatively poor impact strength, optical properties and organoleptic properties—but do have very good tear strength. Conversely, films prepared from homogeneous resins have excellent impact strength, optical properties and organoleptic properties—but poor tear strength. Previous attempts to utilize resin blends to eliminate this problem have not been completely successful. Simple blends of heterogeneous resins with homogeneous resins provide films with sub-optimal organoleptic properties (presumably because of the “grease” fraction in the conventional heterogeneous resin).
Another attempt to solve this problem is by preparing blends of more than one homogeneous resin is disclosed in U.S. Pat. Nos. 5,382,630 and 5,382,631 (Stehling et al., to Exxon). Stehling et al. '631 teach blends which are characterized by the substantial absence of blend components having a higher molecular weight and a lower comonomer content than other blend components (e.g. the high molecular weight homopolymer of conventional heterogeneous resins). These blends are shown to be useful for the preparation of structures having improved tear properties. However, the disclosure of this patent is silent on the subject of impact properties and we have observed that films made from such blends may suffer a very large loss of dart impact strength when the amount of lower molecular weight, higher density component is sufficient to enhance tear strength. One of us disclosed a dual reactor solution polymerization process to prepare a homogeneous copolymer composition which is useful for the preparation of films (Brown, U.S. Pat. No. 6,372,864). None of the inventive copolymer compositions disclosed in Brown '864 contained the high molecular weight, high density (very low comonomer) fraction which is an essential element of the compositions of this invention.
Thus, films prepared from conventional heterogeneous resins have comparatively poor impact strength, optical properties and organoleptic properties—but do have very good tear strength. Conversely, films prepared from homogeneous resins have excellent impact strength, optical properties and organoleptic properties—but poor tear strength. Previous attempts to utilize resin blends to eliminate this problem have not been completely successful. Simple blends of heterogeneous resins with homogeneous resins provide films with sub-optimal organoleptic properties and optical properties (presumably because of the “grease” fraction in the conventional heterogeneous resin).